import base


class CopelandVoting(base.AbstractVotingMethod):
  """Implements Copeland's method."""

  def __init__(self):
    pass

  def name(self) -> str:
    return "copeland"

  def run_election(self, profile: base.PreferenceProfile) -> base.RankOutcome:
    assert self.is_valid_profile(profile)
    copeland_scores = {}
    alternatives = profile.alternatives
    m = len(alternatives)
    margin_matrix = profile.margin_matrix()
    for r in range(m):
      alternative = alternatives[r]
      num_majority = (margin_matrix[r] > 0).sum()
      # Subtract one because we don't include the diagonal.
      num_ties = (margin_matrix[r] == 0).sum() - 1
      copeland_scores[alternative] = num_majority + 0.5 * num_ties
    sorted_scores = sorted(copeland_scores.items(), key=lambda item: item[1],
                           reverse=True)
    outcome = base.RankOutcome()
    outcome.unpack_from(sorted_scores)
    return outcome
